FIG. 1 schematically illustrates a technical concept representing a conventional automatic speed changing apparatus which has been employed for a passenger car, a truck or the like vehicle.
As shown in FIG. 1, the automatic speed changing apparatus is usually constructed such that the content of actuations of a select lever device 10 and an acceleration pedal 20 is inputted into a controller 30 comprising computers 31 and 32, and a rotational speed of an engine 40, a manner of turning on or off a clutch 50 and a manner of shifting gears in a transmission 60 are controlled to match the content of actuations of the select lever device 10 and the acceleration pedal 20.
Here, the select lever device 10 is such that a select lever signal representative of the content corresponding to the actuation position is outputted in response to shifting actuation of a select lever 11 in the direction of an arrow mark Fl in the drawing. For example, when the select lever 11 is actuated to a "R (reverse)" position, a select lever signal representative of "rearward movement of a vehicle movement of the vehicle in the rearward direction)" is outputted, when it is actuated to a "N (neutral)" position, a select lever signal representative of "parking and initiating of engine start" is outputted and when it is actuated to a "D (drive)" position, a select lever signal signal representative of "forward movement" is outputted from the select lever device 10. Particularly, when the select lever 11 is located at the position "D" representing "forward movement", the engine 40 (precisely speaking, an electronic governor 41 incorporated in the engine 40) and the transmission 60 (precisely speaking, a solenoid valve 61 usable for the purpose of shifting gears) are controlled via the controller 30 so that automatic speed changing is effected within the range of, e.g., second speed to fifth speed.
When the select lever 11 is actuated to a position representing "HOLD", a select lever signal representing "maintaining the gear position in the transmission 60 at this time point" is outputted from the select lever device 10. In this case, the gear position which has been selected via the solenoid valve 61 at this time point is maintained in the transmission 60. Namely, the vehicle is then caused to move while a constant speed stage which has been selected is maintained.
In addition, when the select lever 11 is actuated to the position "HOLD", actuation of the select lever 11 in the direction of an arrow mark F2 in the drawing makes it possible to perform a manual speed changing operation. If the select lever 11 is actuated in the direction of, e.g., "DOWN", a select lever signal representing "manual shift down" is outputted from the select lever device 10, whereby the transmission 60 is brought in meshing engagement with speed reduction gears. If the select lever 11 is actuated in the direction of "UP", a select lever signal representing "manual shift up" is outputted from the select lever device 10, whereby the transmission 60 is brought in meshing engagement with speed increase gears.
In connection with the select lever device 10, reference numeral 13 designates an inching switch for commanding movement of the vehicle at a very low speed by a depressing operation without actuation of an acceleration pedal, reference numeral 14 is an "ECON (economy)" switch for commanding so-called economic movement of the vehicle by a depressing operation, reference numeral 15 does a "PWR (power)" switch for commanding so-called powerful movement of the vehicle (movement of the vehicle with high output) by a depressing operation and reference numeral 16 does a "1st (first)" switch for commanding start of movement of the vehicle at a first speed at the "D" position of the select lever 11 by a depressing operation.
FIG. 2 is a flowchart which summarily illustrates a manner of performing the aforementioned controlling operation in relation to the hold processing of the automatic speed changing apparatus (particularly, the controller 30), i.e., a processing to be performed when the select lever 11 is actuated to the "HOLD" position.
As shown in FIG. 2, with the automatic speed changing apparatus as described above, the hold processing is executed under conditions that speed changing is controlled unless the select lever 11 is actuated to the "HOLD" position and the select lever 11 is actuated by no means in the direction of the arrow mark F2 once it has been actuated to the "HOLD" position. The hold processing permits the current speed stage of the transmission 60 (which has been selected at this time point) to be maintained, as mentioned above.
When the select lever 11 is shifted to the "R" position during the hold processing, i.e., when movement of the vehicle is shifted from F (forward movement) to R (rearward movement), the hold processing is naturally canceled, whereby automatic speed changing is performed while the "rearward movement" state is maintained, unless the select lever 11 is actuated to the "HOLD" position again.
When the select lever 11 is actuated further in the direction of the arrow mark F2 while it is actuated to the "HOLD" position, "manual speed changing" is performed in the direction of shift-down or in the direction of shift-up in correspondence to the direction of actuation of the select lever 11, as mentioned above.
Since the automatic speed changing apparatus assures that movement of the vehicle can be excellently stabilized by virtue of the hold function provided in that way and moreover fatigue of a driver (operator) can be minimized, the automatic speed changing apparatus is very significant for a passenger car, a truck or the like vehicle of which movement is comparatively minimally shifted from forward movement to rearward movement and vice versa but employment of the automatic speed changing apparatus is still problematical for a construction machine such as a wheel loader or the like which is designed and constructed in the form of a vehicle having tires used therefor in the same manner as the aforementioned vehicles.
Specifically, since a construction machine such as a wheel loader or the like is such that its movement is often shifted from F (forward movement) to R (rearward movement) and vice versa, as will be apparent from consideration of its utilization, employment of the automatic speed changing apparatus for the construction machine leads to the following malfunctions. Accordingly, work can not be conducted at a high efficiency in spite of employment of the automatic speed changing apparatus. (1) Although the automatic speed changing apparatus can be held at a specific speed stage suitable for conducting a certain work, this hold state should be canceled soon (per every shifting operation from F to R and vice versa). (2) Once the hold state is canceled, it is required that movement of the vehicle is once shifted up to the preceding speed stage and then a hold processing (representing a processing of actuating the select lever 11 to the "HOLD" position) is performed again in order that the automatic speed changing apparatus is held at the preceding speed stage again. (3) A holding processing to be repeatedly performed per every shifting operation from F to R and vice versa is very complicated and incorrect operation is liable to occur.
The present invention has been made with the foregoing background in mind and its object resides in providing an automatic speed changing apparatus for a vehicle in the form of a construction machine such as a wheel loader or the like which assures that a hold processing can always be executed at a high efficiency while the work load of the operator (driver) is reduced.